A boosted engine provides more output power than a naturally aspirated engine of equivalent displacement. In motor vehicles, therefore, boosting can provide a fuel-economy benefit due to the increased power-to-mass ratio of the engine, especially when an exhaust-driven turbocharger provides the boost.
In addition, a variable-displacement engine (VDE) may provide a fuel-economy benefit relative to a fixed-displacement engine of equivalent output power. In this case, the benefit derives from leaving some of the cylinders unfueled and operating the remaining cylinders at higher load, where fuel-economy is improved. In gasoline engines, variable displacement provides additional fuel economy benefits owing to reduced throttling losses and greater tolerance to exhaust recirculation (EGR).
In principle, a variable-displacement engine can be turbocharged to combine the fuel-economy benefits of both approaches. However, the combined benefit may be difficult to achieve across a wide range of operating conditions. In engines that use a single turbocharger, for example, that turbocharger must be configured for the maximum engine load and exhaust flow rate. At reduced exhaust flow, such a turbocharger may provide inadequate boost pressure. This issue has been addressed, in part, by exchange of the single turbocharger for two smaller turbochargers arranged in parallel. In one particular approach, an exhaust valve is arranged upstream of one of the turbines. Closure of the exhaust valve blocks exhaust flow through that turbine during low-load conditions, causing the other turbine to receive all of the exhaust flow. When the load increases to an appropriate level, the exhaust valve is opened, allowing both turbines to share the exhaust flow. In this manner, both turbines can operate efficiently. However, the inventors herein have recognized a disadvantage in the above approach, in that the exhaust valve must be configured for very high-temperature operation. This increases the cost and complexity of the engine system, and may also require additional diagnostics to check for exhaust valve degradation.
To address this issue and provide still other advantages, one embodiment provides an engine comprising a first turbine fluidically coupled to a first group of adjacent cylinders, and a second turbine fluidically coupled to a second group of adjacent cylinders. In this engine, the cylinders of the first and second groups are arranged along a line. The engine also includes a variable valve-lift system configured to admit at least air to the first group of cylinders during reduced engine-load conditions, but to stop admitting air to the second group of cylinders during the reduced engine-load conditions. In this manner, a motor-vehicle engine can enjoy the combined fuel-economy benefits of turbocharging and variable displacement, but without the disadvantages of a high-temperature exhaust valve. Further benefits will be evident from reading the balance of this disclosure and studying the attached drawing figures.
The statements above are provided to introduce a selected part of this disclosure in simplified form, not to identify key or essential features. The claimed subject matter, defined by the claims, is limited neither to the content above nor to implementations that address the problems or disadvantages referenced herein.